Puppies should not be separated from their mother before they are 8 weeks old. Puppies who leave their mothers sooner have a rougher time adjusting and a higher incidence of illnesses. I do not know if it is due to weakened immunity or mourning the premature loss of its family. Their mother's milk provides them with the nutrition and antibodies they need to become healthy dogs. At three to four weeks, puppies should begin eating some solid food. You can try mixing three parts food with one part water or puppy replacement milk . This will make the food easier for the puppy to digest. If your puppy begins eating a little solid food before they leave their mother they will have an easier time adjusting when you bring them home. One way to tell if a puppy is ready to come home with you is if it prefers human company over their mom or siblings.